The invention is particularly directed to a reactor for the production of epitaxial layers on large semi-conducting wafers. There are shortcomings that exist with the commercially available reactors such as discussed in Thin-film polycrystalline Photovoltaic P. 43-83. Final report January 1982, SER/TR-0-9100. Of the four reactor designs, the barrel and pancake reactors are currently favored for the production of epitaxial layers. Of these two designs, the barrel reactor is the most popular for its higher capacity of nominally large wafers, its clean environment and its capability to produce nearly slip free wafers. The present designs of horizontal, barrel and pancake reactors have evolved as scale-ups of previous designs that had nominally large capacities of small wafers, e.g., U.S. Pat. Nos. 3,424,629 and 3,699,298; the reactor capacities were between 30 and 60 wafers of 1.25 inches in diameter; this is equivalent to a maximum processed area of 74 in.sup.2. In contrast, the largest commercial reactor currently available, which is the AMI barrel (Table 1), can hold 24 wafers of 4 inches in diameter; this is equivalent to 302 in.sup.2 of processed area. The realistic epitaxial typical layer thickness tolerances for the barrel reactors are between .+-.5% to .+-.6%. In general, for a given reactor tube diameter, the tolerance increases as larger diameter wafers are fitted into the reactor. In general, conventional barrel reactors produce a thicker deposit on the side edges of the wafers as compared to the center. In some cases the thinnest deposit is between the center and the edges of the wafer. The theoretical reason is that as the wafer diameter increases, the ratio of the distance from the center of the wafer surface to the interior of the round reactor tube wall and the corresponding distance from the side edges of the wafer surface to the tube wall become more unequal. Hence, the mass transport diffusion distance from the gas stream to the side edges of the wafer surfacer is shorter than it is at the center. The result is a thicker epitaxial deposit at the edges than at the center. In order to reduce this problem, the reactor tube diameter should be increased as the wafer size is increased. Although larger barrel reactors, with capacities of up to 21 wafers of 6 inch diameter, are in the development stage, the anticipated purchase cost is in excess of $1,000,000 per unit. Also, since the expense of operating large barrel reactors increases rapidly with size, and in general, the reliability decreases rapidly with size, the economics of such a system are questionable.
The Pancake reactor systems, in general, operate more reliably but are more difficult to scale up for large wafers. Such systems are capable of producing epitaxial layer thickness tolerances of .+-.3%. Since gases are recirculated over the product wafers, and since the induction heating coil is mounted inside the reactor tube, the integrity of the gas ambient with respect to total amount of impurities is not as good as a barrel. Also, it is more difficult to grow slip free epitaxial layers in a pancake reactor because it is more difficult to reflect radiant heat energy onto the front surfaces of the wafers. Nevertheless because of high reliability and tight tolerances, pancake reactors are a popular choice where product specifications can tolerate the drawbacks.
With reference to Table 1 and the previous discussion, it should be evident that the bottleneck for producing epitaxial layers on large wafers is the designs of present CVD reactors and their cost verses thru put. Current crystal growth technology can produce high quality 8 inch diameter wafers. Since the history of semiconductor processing has shown that overall economics are in favor of handling the largest wafers possible, the industry would like to process the 6 to 8 inch diameter wafers that are now available. At present, a cost effective and reliable barrel or pancake reactor that can process these wafers cannot be envisioned.
Metal organic chemical vapor deposition (MOCVD) of III-V compounds is taking the path of silicon epitaxial reactor development for the growth of III-V compound layers on III-V or silicon substrates. Also, the barrel reactor geometry is used for gas plasma etching apparatus. Any improvements made in distributing gases in barrel reactor could possibly be used for such systems. Also, there is the possibility of barrel type plasma deposition systems.